The invention relates to linearization of a radio transmitter.
The scarcity of radio frequencies leads to a need to use spectrum-efficient modulation methods in new radio systems. In Europe, a new radio system standard has been developed for PMR (Professional Mobile Radio) users, called TETRA (Terrestrial Trunked Radio). As the modulation method of the TETRA system, π/4-DQPSK (π/4-shifted Differential Quadrature Phase Shift Keying) has been chosen. From the transmitter point of view, a drawback of the modulation method is the variation in the amplitude of the envelope of the RF signal. In a nonlinear amplifier, such a variation causes InterModulation (IM). IM products spread the spectrum of the transmitted signal, and thus tend to reduce the benefits of using a linear modulation method. The IM products cannot usually be filtered as they form very close to the desired signal. With constant amplitude modulation methods, spectrum spreading does not occur, and consequently the signal may be amplified by a nonlinear amplifier.
A trunked PMR system, in which different user groups share same radio channels, has strict requirements regarding adjacent channel interference caused by a transmitter. These requirements call for a good linearity in the transmitter of the used radio system.
In a power amplifier, good linearity is only achieved with poor efficiency. However, the efficiency of portable devices should be as high as possible for the operation time to be adequate and in order not to waste battery capacity. Further, at least relatively good efficiency is required of the power amplifiers at base stations to avoid cooling problems. The achieving of adequate efficiency and linearity calls for linearizing the transmitter.
If the nonlinearities of an amplifier were known in advance, it would be possible to form inverse functions of the nonlinearities, and use them to convert the input signal, whereby the nonlinearities would be cancelled. The properties of the amplifier do not, however, remain the same but vary due to ageing, warming up, and according to the radio channel and transmit power used. In addition, the amplifiers have individual differences. Need exists for linearization methods that must in an adaptive way be capable of adapting to changing conditions.
Development work has been targeted into a number of different linearizing methods, and three of them have been found to possess qualities suitable for practical radio systems. These methods are feedforward, cartesian feedback and predistortion. A linearization method can also be adaptive.
Thus, if the nonlinear transfer function of the amplifier is known and if it does not vary as a function of time, the signal to be transmitted can be linearized by applying a suitable transfer function causing a predistortion to the signal. This way, the signal coming out of the amplifier can be made linear. This method is called predistortion. Predistortion is usually performed on baseband using a lookup table into which the conversion parameters causing the predistortion, i.e. the predistortion parameters, are stored.
For instance, with the temperature and age change of the amplifier its transfer function also changes and it becomes necessary to update the predistortion parameters in the lookup table. Prior art solutions use a gradient-based search for this purpose. A gradient-based search adapts towards minimizing an error. A problem in a gradient-based search is its slowness.